In an adaptive immune response, T lymphocytes are capable of recognising internal epitopes of a protein antigen. Antigen presenting cells (APC) take up protein antigens and degrade them into short peptide fragments. A peptide may bind to a major histocompatability complex (MHC) class I or II molecule inside the cell and be carried to the cell surface. When presented at the cell surface in conjunction with an MHC molecule, the peptide may be recognised by a T cell (via the T cell receptor (TCR)), in which case the peptide is a T cell epitope.
T cell epitopes play a central role in the adaptive immune response to any antigen, whether self or foreign. The central role played by T cell epitopes in hypersensitivity diseases (which include allergy, autoimmune diseases and transplant rejection) has been demonstrated through the use of experimental models. It is possible to induce inflammatory or allergic diseases by injection of synthetic peptides (based on the structure of T cell epitopes) in combination with adjuvant.
By contrast, it has been shown to be possible to induce immunological tolerance towards particular peptide epitopes by administration of peptide epitopes in soluble form. Administration of soluble peptide antigens has been demonstrated as an effective means of inhibiting disease in experimental autoimmune encephalomyelitis (EAE—a model for multiple sclerosis (MS)) (Metzler and Wraith (1993) Int. Immunol. 5:1159-1165; Liu and Wraith (1995) Int. Immunol. 7:1255-1263; Anderton and Wraith (1998) Eur. J. Immunol. 28:1251-1261); and experimental models of arthritis, diabetes, and uveoretinitis (reviewed in Anderton and Wraith (1998) as above). This has also been demonstrated as a means of treating an ongoing disease in EAE (Anderton and Wraith (1998) as above).
The use of tolerogenic peptides to treat or prevent disease has attracted considerable attention. One reason for this is that it has been shown that certain tolerogenic epitopes can down-regulate responses of T cells for distinct antigens within the same tissue. This phenomenon, known as “bystander suppression” means that it should be possible to induce tolerance to more than one epitope (for example, all epitopes) within a given antigen, and to more than one antigen for a given disease, using a particular tolerogenic peptide (Anderton and Wraith (1998) as above). This would obviate the need to identify all of the pathogenic antigens within a particular disease.
Peptides are also a favourable option for therapy because of their relatively low cost and the fact that peptide analogues can be produced with altered immunological properties. Peptides may thus be modified to alter their interactions with either MHC or TCR.
One possible problem with this approach is that it has been shown that not all peptides which act as T cell epitopes are capable of inducing tolerance. The myelin basic protein (MBP) peptide 89-101 is an immunodominant antigen after immunisation and is also a very effective immunogen both in terms of priming for T cell reactivity and induction of EAE. However, this peptide has been shown to be ineffective at inducing tolerance when administered in solution (Anderton and Wraith (1998), as above).
A number of explanations for the observed hierarchy in the ability of T cell epitopes to induce tolerance have been proposed (reviewed in Anderton and Wraith (1998) as above). In particular, it has been proposed that there is a correlation between the affinity of the peptide for the MHC and tolerogenicity (Liu and Wraith (1995) as above), but this does not tally with some of the observations. For example, MBP[89-101], which is not tolerogenic, binds to I-AS with relatively high affinity. It is thus not straightforward to predict which peptides will induce tolerance.
The present inventors have shown that if a peptide epitope is of an appropriate size to be presented by immature APC without antigen processing, it can induce immunological tolerance (International patent application number PCT/GB01/03702). The observation that some T cell epitopes are tolerogenic and others are incapable of inducing tolerance can therefore be explained by the fact that some epitopes require further processing before they are capable of being presented by an MHC molecule. These epitopes which require further processing do not induce tolerance when administered in a soluble form, despite their capacity to induce disease when injected in combination with adjuvant.
The epitopes which do not require further processing are capable of inducing tolerance, and have been termed “apitopes” (Antigen Processing Independent epiTOPES) by the inventors.